The present invention generally relates to transmit power control, and particularly relates to determining transmit power control feedback.
Transmit power control plays an important role in interference-limited communication networks, such as those based on Code Division Multiple Access (CDMA) technologies. Reliable communication and targeted levels of data throughput require transmission at sufficient power to insure adequate received signal quality, but transmitting at excess power is avoided as a mechanism to limit or otherwise reduce interference.
As one example of transmit power control, a first transceiver transmits an information signal to a second transceiver, and the second transceiver transmits power control feedback to the first transceiver as a function of received signal quality, as measured by the second transceiver for the information signal. In turn, the first transceiver increases or decreases transmit power for the information signal in response to the power control feedback. In this manner, the transmit power moves up and down, often within an allowable or otherwise bounded range, as needed to keep the received signal quality at the second transceiver at or about the targeted level over changing reception conditions.
Commonly, the power control feedback comprises Transmit Power Commands (TPCs) transmitted as 1s or −1s, depending on whether the measured signal quality is above or below a reference target. Such control often is referred to as “inner loop” power control and, as the term suggests, an “outer loop” power control mechanism often is paired with inner loop power control. With outer loop power control, one or more additional metrics, such as Bit Error Rate (BER) or Frame Error Rate (FER), or Block Error Rate (BLER), provide the basis for adjusting the inner loop target. That is, the inner loop power control generates TPCs by comparing measured signal quality to a target value, and outer loop power control adjusts the target value by comparing the additional metric(s) to corresponding target values, e.g., an FER or a BLER target of one percent.
Some contexts complicate the above approach to transmit power control. For example, the Wideband CDMA (W-CDMA) standards call out the use of downlink associated dedicated physical channels (ADPCHs) to send TPCs to user equipment (UE), to ensure that the UE transmits certain uplink control channels at transmit powers that result in the base stations receiving those channels at targeted signal qualities. For example, in the extension of W-CDMA systems denoted High Speed Downlink Packet Access (HSDPA), High Speed Dedicated Physical Control Channels (HS-DPCCHs) used by UE to signal Acknowledgement or Negative acknowledgement (ACK/NACK) for Hybrid automatic repeat request (H-ARQ) operation of the High Speed Downlink Shared Channel (HS-DSCH) generally are power-controlled by the network to ensure reliable reception by the supporting network base station(s). In turn, the UE returns TPCs to the transmitting network to ensure that the downlink TPCs are transmitted to the UE with sufficient power for reliable reception. In other words, the UE sends transmit power control feedback for the downlink power control channels to ensure that the UE receives the network-transmitted TPCs at a targeted signal quality.
Pilot information is included in the DPCH transmissions, e.g., one pilot symbol per time slot, and the receiving UE can use the received pilot information to estimate DPCH signal quality for generation of per-slot power control feedback. That is, the UE generates uplink TPC commands as feedback for the received DPCH by comparing downlink signal-to-noise-plus-interference ratio (SINR) measurements with a target SINR, which is set by the outer-loop power control.
However, to support greater numbers of HS-DSCH users without requiring additional downlink DPCHs for each user, the W-CDMA standards identify the use of “Fractional Dedicated Physical Channels” (F-DPCHs), which time multiplex a number of DPCHs for different UE onto one downlink channel. While this methodology consumes fewer spreading code resources on the downlink, it does complicate transmit power control at the UE because the F-DPCHs do not include per-slot pilot information on which UEs could base their signal quality estimation. Nor does the F-DPCH provide enough data symbols per slot to support accurate signal quality estimation from the received data symbols. Thus, as a non-limiting example, the F-DPCH exemplifies the type of channel that complicates inner/outer loop power control.